Over the years many types of systems have been developed for controlling the pneumatic movement of solid particulates from one location to another. Examples of material moved in such a manner are grain, plastic pellets, sand, fly ash, cement, powdered coal, carbon black, titanium dioxide, fluid catalytic cracking catalyst, and other particulate solids. Typically, the apparatus employed for controlling the movement of these solid particulates uses a continuous revolving positive displacement type valve or screw feeder, a pinch valve on a timer, or blow pots which discharge the solid particles from a relatively dense system into a pneumatic transport medium so that the particles can be conveyed to another area. In units for the fluid catalytic cracking of hydrocarbons the movement of particulate solids between the reactor and regenerator is controlled by slide valves. However, the use of slide valves where good control is desired is limited to fairly high flow rates, in the range of twenty tons per hour or higher. If accurate continuous metering or measuring of the rate is desired it is common practice to employ some type of weigh system, such as, load cells or actual scales (balances), but normally there is no closed loop control of these functions, as in the system of the present invention. Also, up to now there has not been a reliable system for continuously controlling and metering a specific amount of hot particulates from one location to another pneumatically.
The continuous measurement and control of small amounts of pneumatically transportable fluidizable particulates has typically been a problem. The industry has been plagued with the problem of maintaining a continuous flow of particles in a fluidized transportable state in small lines of less than 6" in diameter without bridging and slugging. This problem has resulted in solutions that are not readily adaptable to continuous measurement or control. An example of this difficulty is readily apparent if one considers the history of the pneumatic transport of ambient fresh and hot regenerated equilibrium catalyst to and from a fluid catalytic cracking (FCC) regenerator.
Since the 1940's oil refiners have been operating fluid catalytic cracking process units (FCCU's) to convert heavy oils to gasoline and olefins for alkylation or petrochemical feedstocks. Up until the energy crisis of 1970's, most refiners were operating their FCC units on high quality virgin gas oils that did not contain any metals or very little metals. This resulted in most refiners adding the fresh FCC catalyst on a batch basis at a rate close to or slightly higher than the catalyst loss rate from the unit. In other words, for most refiners the amount of fresh catalyst added resulted in very little increase in the unit inventory, so the amount of equilibrium catalyst withdrawn from the unit, typically from the regenerator to an equilibrium catalyst storage hopper, was minimal. Typically, the refiner withdrew a small amount of equilibrium catalyst once a week to maintain the inventory and to provide equilibrium catalyst for upsets and start-ups. Since the mid 1970's more and more refiners are processing residual oil in their FCCU's as a method of increasing their transportation yields at the expense of heavy fuel oil. These residual oils contain metals such as vanadium and sodium, which act as FCC catalyst poisons to lower activity, and nickel, which acts as a dehydrogenation catalyst which is harmful to the FCC yield structure. To reduce the effect of these metals on the catalyst activity and selectivity, refiners that process residual oil add more fresh catalyst and therefore need to withdraw more equilibrium catalyst.
As an example, refiner operating on clean gas oil feedstock may add 0.1 to 0.15 pounds of fresh catalyst per barrel of feed to maintain activity and selectivity. With normal cyclone operation in the reactor and regenerator, this particular refiner would be withdrawing from about 0.01 to 0.03 pounds of equilibrium catalyst per barrel of FCCU feed. On the basis of a 50,000 BPD unit, this would amount to 2.5 to 3.75 tons per day of fresh catalyst addition, with 0.25 to 0.75 tons per day of equilibrium catalyst withdrawn. At an average total catalyst inventory in an FCCU of 10 pounds per barrel per day of capacity, 0.25 to 0.75 tons per day amounts to an increase of 0.1% to 0.3% of the total unit catalyst inventory per day.
If this same refiner added residual oil to his FCC feed, the fresh catalyst addition rate might be 1.0 pound per barrel of feed or higher. This then requires a fresh catalyst addition rate of 25 tons per day and a withdrawal rate of about 20 tons per day, or approximately 8% of the inventory per day.
In the early 1970's, it became apparent to some persons in the industry that batch addition of fresh catalyst to the FCCU was resulting in operational swings that were detrimental to unit performance. Also, in the mid 70's with the advent of oxidation promoters and FCC catalytic additives in the early 1980's, the industry started to look for better ways to continuously add these fluidizable particles. Today, the typical FCC unit is designed with some type of "continuous" fresh catalyst addition system, such as, a weigh hopper, a continuous revolving positive displacement valve, a cycled on-off type valve or a pinch valve to control the flow of ambient temperature fresh FCC catalyst from the fresh catalyst storage hopper to the FCC unit regenerator using transport air. The two main drawbacks of these systems is that they are batch systems and they do not really control the rate of addition on a continuous basis so that they cannot be readily automated to control a process variable.
All of the "continuous" systems described above are for ambient temperature and relatively small storage type hoppers that are easily adaptable to load cells but are not usable for handling materials heated to greater than about 400.degree. F. So until now, there has not been any system for continuously controlling the hot equilibrium FCC catalyst withdrawn from the regenerator to control the unit catalyst inventory. Typically this is a batch operation which is normally manually controlled with only a 4 inch gate valve located on the regenerator vessel. On a small amount of catalyst, this withdrawal is usually done once a week since it is not easily controlled, but on residual oil operations it is a daily event.
The withdrawal of large and small amounts of hot equilibrium solid from a fluid regenerator creates a number of problems for the refiner in operating the unit. It creates mechanical and maintenance problems, and creates the potential for injury to operating personnel. In the normal operation of withdrawing the equilibrium catalyst from the regenerator to a catalyst storage hopper, the hopper is operated under vacuum to aid in transporting the hot particulates, and transport air is added to fluidize and transport the particulates from the pressurized regenerator into the lower pressure storage hopper. The control of the withdrawal rate from the regenerator is normally performed by manual control through a 4" gate valve. The gate valve is normally located directly on the side of the regenerator on a regenerator vessel nozzle that is angled down at 45.degree. or greater from the horizontal. Just downstream of the gate valve transportation air is injected to lower the catalyst density and transport it through a 4" line into the storage hopper Those knowledgeable in the operation of these type of systems are familiar with the problem of control with using a 4" gate valve. First of all there is no way to measure the rate, and secondly, the inconsistencies inherent with particulate flow result in large fluctuations in flow rate at the same valve opening, and typically the flow may stop all together. Another problem inherent in this type of operation is the inability to withdraw the exact amount of solid desired. Typically, the operator opens the transport air and regulates the 4" gate valve to establish some particulate flow and then goes about his other duties. If he gets busy, the flow may stop or he may not return to stop the flow until he is made aware of a problem by the board operator, or until he has withdrawn more than the desired amount of particulate. Another problem inherent in this type of system solved by the unique system described here is that because of poor flow control the pipe between the regenerator and storage hopper is subjected to tremendous fluctuations in operating temperature from the transport medium temperature sometimes to within a 100.degree. F. of the regenerator operating temperature, and it may be operated at higher than desired carrying air rates, both of which shorten the life of the system.
In units which withdraw large amounts of particulates over a short period of time, rather than continuously, there is the possibility of mechanical damage to the receiving truck or rail car that removes the equilibrium particulate stream from the site. Withdrawing large amounts of catalyst at one time results in very little cooling of the particulate stream before it reaches the hopper. In fact the temperature of the withdrawn catalyst might be only 100.degree.-300.degree. F. lower than the regenerator temperature, or in the range of 1400.degree.-1100.degree. F., when it reaches the storage vessel. Since the particles are a very good insulator, it is possible to have material in the hopper that is hotter than the 250.degree. F. limit placed on trucks, so that when the particulates are unloaded from the hopper to the truck, the truck is damaged. The present invention provides a system for controlling the rate of particulate flow coupled with the installation of a catalyst cooling system, such as a finned tube section of pipe installed in the catalyst withdrawal line, which will eliminate the problems inherent in the presently used systems.
Up until now there has not been a good method developed for controlling the flow of hot solid particulates, such as hot regenerated fluid catalytic cracking catalyst, regenerated heat transfer medias such as those employed in the processes described in patents directed to the ART Process (described in U.S. Pat. No. 4,263,128) and the 3D Process (described in U.S. Pat. No. 4,859,315), and other hot solid particulates from one system to another at low rates, such as 1 #/day up to 200 tons per day or higher. The system described below now enables the operators of fluidized solid process systems such as the fluid catalytic cracking process, 3D Process, MSCC Process (described in U.S. Pat. No. 4,985,136) and DEMET Process (described in U.S. Pat. No. 4,686,197) and other fluidized particulate type processes to continuously control and measure the rate of moving hot regenerated or spent catalyst solids from the one location to another. The present system is also applicable to controlling and measuring the underflow from third stage cyclones to disposal or final clean-up, and for unloading electrostatic precipitator particulate recovery hoppers. Also, the present system can be used to add fresh catalyst or other solids from storage hoppers into the regenerator. In general, the process and apparatus of the present invention can be used to control a precise amount of any fluidizable solid to be moved from one point to another, provided there is a sufficient pressure differential and transport medium to provide the force needed to move the particulate solid.
In particular, the present invention allows the operator to control and measure the movement of regenerated solid particles from, for example, an FCC or 3D regenerator to a storage hopper or another process system, such as DEMET. The present invention is applicable to the movement, control and measurement of fluidizable particulate solids at any temperature above or below the temperature of the transport media and at any rate up to 20 tons per hour or more. The pressure is not critical, except that the pressure of the source system plus the head of fluidizable solid to be conveyed must be greater than the system pressure to which the fluidizable solid is being conveyed.
Therefore, a primary object of the present invention is an improved method of controlling or metering particulate solids to be pneumatically transported from one location to another. A related object of the present invention is improved apparatus for such controlling or metering of particulate solids. Still a further object of the invention is an improvement in a process system wherein a fluidized particulate solid is moved between locations, which provides a more accurate control and measurement of the amount of particulate solid being transported over a period of time. Other objects and advantages of the present invention will become apparent from the following description thereof and from the practice of the invention.